Guided by argumentation schema theory, we conducted five psychological studies in the United States and China on arguments about vaccination. Study 1 replicated research about arguments on several topics, finding that agreement judgments are weighted toward claims, whereas quality judgments are weighted toward reasons. However, consistent with recent research, when this paradigm was extended to arguments about vaccination (Study 2), claims received more weight than reasons in judgments about agreement and quality. Studies 3 and 4 were conducted in the United States and China on how people process counterarguments against anti-vaccination assertions. Rebuttals did not influence agreement but played a role in argument quality judgments. Both political position (in the United States) and medical education (in China) predicted differences in argument evaluation. Bad reasons lowered agreement (Study 5), especially among participants studying health care. Political polarization apparently heightens the impact of claim side in the argumentation schema, likely to the detriment of public discourse.

Argumentation schema theory guided four experiments on the processing of plausible and implausible reasons and warrant statements testing the hypothesis that most reasons produce greater agreement with claims than when claims are presented without support. Another hypothesis was that leaving warrants unstated often produces greater agreement than when the warrant is made explicit. In Study 1, American participants were more likely to agree with claims after they read arguments than beforehand—even those with implausible reasons and warrants. In Study 2, American history and environmental science majors read brief arguments and agreed more with implausible arguments than claims alone. Study 3, with Chinese participants, replicated some but not all earlier results. In Study 4, with Chinese participants, blatantly false claims supported by bogus reasons yielded marginally greater agreement than unsupported claims. These findings suggest that many people have uncritical argumentation schemata with low support thresholds, making them vulnerable to weak and bogus arguments.